Extensometer



Dec. 22, 1953 A. c, RUGE 2,663,085

EXTENSOMETER Filed Dec. 1, 1951 'disengageable clip Patented Dec. 2 2, 1953 UNITED, STATES PATENTI OFFICE EXTENSOMETER Arthur 0. Huge, Cambridge, Mass., assignor to .Baldwin-Lima-Hamilton Corporation, a corporation of Pennsylvania Application December 1, 1951, Serial No. 259,386

11 Claims. (01. 33-147) 1 This invention relates generally to electrical extensometers of a type employing a bending beam which flexes in response to specimen strains to be measured and on which is mounted electrical means for determining the extent of flexure.

One object of my invention is to provide an improved electrical strain sensitive bending beam type extensometer which is so constructed, ar-

ranged and operated that a comparatively small and light weightbending beam may be employed and efiectively attached to a specimen in a relatively simple, expeditious and yet positive manner withvminimum possibility of injury to or dis tor-tion of the beam. 7

Another object isto provide an improved electrical strain sensitive bending beam type extensometer in which a very positive specimen gripping action of the extensometer can be efiected independently of the bending beam.

Further objects are to provide an improved electrical strain sensitive bending beam type extensometer in which bending beams of difierent .capacities may be used in an extremely simple but effective manner for measuring either large or small strains; to provide a bending beam arrangement capable of either averaging strains in a specimen electrically or. of. measuring the bending strains; to provide electrical strain sensitive bending beam structure that is readily separable from the extensometerupon failure of the specimen; and to accomplish these and other objects while maintaining a high degree of sensitivity,

accuracy,. and stability during, repetitive opera- Other objects and advantages will be more apparent to those skilled in the art from the followingdescription of the accompanyingdraw- 'ings' in which:

ping head;

" Fig. 2 is'a perspective of the extensometer viewed from the front'thereof, showing the test specimen ruptured at, the end of the tension test and with one of the clip gages automatically disengaged from the gripping head upon occurrence ofthe rupture;

.the extensometer the pairs of knife edges so as to attach thegrip- Ipingheadto. and to remove it from, the test P-in9 1 Fig. 4 is a perspective of one of the clip gages removed from the extensometer. v

In the particular embodiment of the invention which is disclosed herein merely for the purpose of illustrating on specific form among possible others that the invention might take in practice, I have shown in Figs. 1 and 2 a gripping frame, generally indicated at I 0, comprising similar upper and lower heads II and I2. Hence, the description of one will suffice for both. Each head consists of a pair of parallel members I3 and I4 for supporting clip gages to be described later, and a pair of horizontal plates I5 and I6 shaped to provide a pair of specimen engaging knife edges I1 and I8 respectively, being preferably of the V and straight edge types. A flat clamping spring I9, secured by screws 25 to the rear ends of each pair of supporting members, not only holds each pair of supporting members in spaced relation but also imparts a gripping force to the pairs of knife edges I1 and I8 when they are sprung apart to be applied to a test specimen 2| held betweena pair of conventional specimen holders 22 and 23 of a usual materials testing machine (not shown). The upper and lower heads II and I2 are vertically guided and normally supported relative to each other by vertical lateral guide plates 24 and 25, one of which is secured at one end by screws 26 to lower supporting member I3 and the other at its opposite end by screws 26' to the upper supporting member I I. The other ends of guide plates 24 and 25 are provided with longitudinal slots 21. Thumb screws 28and 28 are threaded in members I3 and I4 and extend through said slots to releasabiy secure the plates to upper and lower members [3 and;I4.;;When these thumb screws are loosened; upper and lower heads II and I2, carrying the pairs of knife edges ll and I8, may be moved vertically relative to each other.

Rotatable stops 29, pivoted to the guide plates, serve, when in vertical position, to engage members I3 and I4 for quickly; and accurately determining, without measurement, a predetermined gage length between the upper and lower pairs of knife edges. During operation of the extensometer, the stops are rotated out of position so that the extensometer can operate in either tension or compression. v I

A pair of clip gages generally indicated at 30 are each provided with sensing elements comprising a bending beamin the form of a flat spring 3|, and electrical wire resistance strain gages 32 bonded in a usual manner to the broad 3 surfaces of spring 3|. In order for these beams to be rendered highly effective as to their functional characteristics and readily attachable to the extensometer heads I provide identically made but reversely positioned upper and lower supporting and bending moment arms 33 and 34 secured by screws 35 to sprin 3| at the ends thereofand posed'substa ti l' a r sht an l s to the spring when positioned in the extensometer. For purposes of length adjustment, each arm is made in two sections 36 and 31 with over-' lapping ends thereof held together by a screw 38 threaded into section 36 through a lengthwise slot in section 31. For mounting the clipgag'es 30 on the extensometer heads H and [2, the outer sides of section 31 at the free ends thereof are provided with points 40 while the inner sides of knife edge plates l5 and I6 are provided with vertically aligned conical depressions 42. When the clip gages are removed from the extensometor the bending beams take the curved shape shown in Fig. 4. Hence; in mounting the gages between heads H and 12 the'freeends of the supporting arms are moved towards each other thereby causing spring 34 to bend until points 40 can he sprung into conical depressions 42. The tension thus given spring 31 tends to spread the points and hold engaged in depressions 42. Straingag lead wires 43' are connected to usual indicating or recording instruments. Stops M formed of suitable stifi wire are rigidly pendently'sup port-ed from the front edges or the upper pair of plates [5 and l fi on both sides of each upper arm 33. These stops serve to keep clip gages 30 substantially towards the front of the gripping head away from each other to prevent entanglement when the clip gages are disengaged ntjm the grippingframe, hereinafter *operd-tionr-The gage length of the knife edges is set by first swinging-members 29 vertically and compressing the upper arid lower' heads it and 12' toward each other to engage members 23, and

'someter is attachedto the test specimen, but

slotted ends to engage screws '26 and 28 on each :s ide ofthe ext-nsemeter heads n and 'W-ith the yoke ends of the tool thus engaging the heads the other ends of the tool are moved together by the operators hand as shown in Fig. "3', causing the opposed knife eases torotate'away from each other and causing clamping springs 19 to beterisioned. When the pairs of knife edges have been th s moved fa-r'enough apart, the extensometer is applied to the test specimen and the tool removed. After loosening thumb screws 23 and moving rotatable stops 2-9 out :of position, clip gag-es 30 are then mounted between" upper and lower heads H and ll-of' the gripping head, in the manner described above. Elongation of the specimen when subjected to a test load causes the upper and lower heads to move vertically apart and the tension initially given bending beams 31 is proportionately reduced thus changing the resistance of thewiregage's. change in resistance is measured by suitable instrument's well known in the art and the eiitensometer would, of course, be pro-calibrated so as to know the extent of specimen strain for given changes 'in resistance; 'When the specimenfina-lly fails,

tightening screws 28; The eatenthe clip gages 30, either simultaneously or sue cessively, may fall out of the gripping head and simply hang freely suspended by their lead wires 43 without damage. The extensometer can, of course, be removed if desired before the point of failure is reached but this is not necessary. It is also to be understood that compression tests the clip gages do not become disengaged.

By employing'the symmetrical arrangement of two bending beams shown in Fig. 1 it will be seen that I can measure the strains of the individual beams independently, from which I can readily calculate both the amount of bending to which the specimen is subjected and also the average strain. In practice, the two bending beams are connected into a single circuit so that the addition orsubtractionrequired for such calculation is performed electrically. The circuits by which such results are obtained are well known and are not considered to be apart of the present inven tion.

It is seen from the above description that I have provided a relatively simple and yet highly effective bending beam typeextensometer having the many desirable advantages previously mentioned as well as others.

It will, of course; be undenstood that various changes details of construct-ion and arrang-ement of parts-may be made by those' skilled inthe art without departing from the spirit of the invention asset forth in the appended claims.

1. An extensomete'r comprising, combination, upper and lower substantially parallel heads, means carried by each head for gripping :a test specimen and for preventingrotaticn of-the heads relative to the specimen attheir respective points of attachment to it. a bending beam disposed laterally of the specimen and-extending ilengthwise thereof, arms rigidly connected to the ends of the beam, said arm-s being disposed in planes substantially normal to the specimen axis, means for connecting the arms to said heads so that movement resulting from specimen strain in a direction lengthwise of the specimen causes the arms to eifect flexure of the beam, and electrical means mountedion the beam soasvto be responsive to 'fiexurethereofas ame'asure of the entent of specimen strain. g h

2 The combination set forth in claim 1 further characterized inthatjeachof the upper and lower heads comprises opposed specimen engaging elements and a spring member vconnecting the opposed elements, said spring members being offset to one side f the en aging. elements so s to b s the s'ame'toward each other to exert a rippin force on a specimelilvvthen attached thereto 3. An extenso'meter comprisinggin combination, upper and lower heads, means carried by each head for .s' ippms a testpec men. d-

in hears-disposed laterally of, the specimen an extending lengthwise thereof, arms rig-idly connected to the ends of the beam, said arms being disposed in planes substantially normal to the specimen axis; means :Ie'r connecting: the arms to "said heads so that movement resisting from specimen strain in a direction leng-iihw-ise of the specimen causes thee-r ms to effect flexure of the beam, and electrical means mounted on thebeam so as to be responsive to sense thereof as a measure of'the extent of specimen strain, said "means for connecting the arms-to the upper and lower heads comprising abutting contacts between the arms and heads and the bending beam being -prestressed so as to bias said contacts into their abutting engagement.

4. An extensometer comprising, in combination upper and lower heads, means carried by each head for gripping a test specimen, a bending beam disposed laterally of the specimen and extending lengthwise thereof, arms rigidly connected to the ends of the beam, said arms being disposed in planes substantially normal to the specimen axis, means for connecting the arms to said heads so that movement resulting from specimen strain in a direction lengthwise of the specimen causes the arms to effect fiexure of the beam,

and electrical means mounted on the beam so as l to be responsive to fiexure thereof as a measure of the extent of specimen strain, said means for connecting the arms to the upper and lower heads comprising abutting contacts between the arms and heads and the bending beam being prestressed so as to bias said contacts into their abutting engagement, said abutting contacts being on the side of said heads facing toward each other whereby, in the event of rupture of a specimen under a tension test, said abutting contacts will automatically separate and release the arms and bending beam.

5. An extensometer comprising, in combination, upper and lower heads, means carried by each head for gripping a test specimen, a bending beam disposed laterally of the specimen and extending lengthwise thereof, arms rigidly connected to the ends of the beam, said arms bein disposed in planes substantially normal to the specimen axis, means for connecting the arms to said heads so that movement resulting from specimen strain in a direction lengthwise of the specimen causes the arms to efiect flexure of the beam, electrical means mounted on the beam so as to be responsive to fiexure thereof as a measure of the extent of specimen strain, and guides supported by one of the heads and projecting down along each side of a beam arm to limit lateral movement thereof.

6. The combination set forth in claim 1 further characterized by the provision of plates extending between upper and lower heads for connecting the same together in adjustable spaced relationship, and means supported on one of said plates for limiting movement of the upper and lower heads toward each other to determine the gage length of the extensometer.

7. The combination set forth in claim 1 further characterized by the provision of a second bending beam and arms rigidly secured thereto for connection to the upper and lower heads on the side of the specimen opposite to that of the other beam and arms, and electrical means mounted on the second beam so as to be responsive to flexure thereof as a measure of the extent of specimen strain.

8. The combination set forth in claim 1 further characterized in that the electrical means mounted on the beam comprises a bond-ed wire type strain gage for measuring the surface strains in the beam.

9. The combination set forth in claim 1 further characterized in that the bending beam is an elongated flat spring whose flat surface is substantially normal to the axes of the arms to which the beam is attached.

10. An extensometer comprising, in combination, a frame having upper and lower heads, opposed specimen engaging means carried by each head for gripping the test specimen and preventing rotation of the heads relative to the specimen at their respective points of attachment to it, a fiat plate spring connecting the opposed specimen engaging means of one head and a similar flat spring for connecting the opposed specimen engaging means of the other head, said springs lying in a substantially common vertical plane offset to one side of the specimen engaging means and adapted to bias the opposed specimen engaging means of each head toward each other for gripping the specimen when attached thereto, and means having separable abutting engagement with said heads and being responsive to movement between said heads upon occurrence of strain in a specimen to which the extensometer is connected.

11. An extensometer comprising, in combination, upper and lower heads, means carried by each head for gripping a test specimen, a bending beam disposed laterally of the specimen and extending lengthwise thereof, arms rigidly connected to the ends of the beam, said arms being disposed in planes substantially normal to the specimen axis, means for connecting the arms to said heads so that movement resulting from specimen strain in a direction lengthwise of the specimen causes the arms to effect flexure of the beam, and electrical means mounted on the beam so as to be responsive to flexure thereof as a measure of the extent of specimen strain, the bending beam and arms therefor having provision for varying the radial length of the arms and thereby varying the sensitivity of the beam for a given strain.

ARTHUR C. RUGE.

References Cited in the file of this patent UNITED STATES PATENTS FOREIGN PATENTS Country Date Great Britain July 23, 1920 Number Number 

